The present invention relates to a constant-current generating circuit and more particularly to a temperature dependent constant-current generating circuit applied to a feed forward type stabilized driving circuit for keeping a light emission characteristic of a light output device composed of a laser diode as a light source such as a optical transmission device or an optical data link.
Optical communications and optical data links have been suddenly widespread in recent years. In the optical transmission device used for these uses generally, a laser diode (LD) is directly modulated in intensity so as to generate a optical signal and the optical signal is transmitted via a transmission medium such as an optical fiber or by propagating through the free space. Particularly, in a transmission module used for a optical data link such as not only a subscriber system such as FTTH (fiber to the home) for use of optical communication at each home but also giga-bit Ethernet, IEEE1394, and optical wiring, the following method is adopted as a method for directly modulating the intensity.
The LD is kept at a DC bias current less than the laser oscillation threshold value in a state where an optical signal is OFF. However, in a state where a optical signal is ON, the LD is brought to a condition where a perfect laser oscillation is obtained, applying a pulse current with an amplitude enough to obtain necessary output intensity, thereby making the intensity ratio of a optical signal in ON and OFF state as large as possible.
When a signal to be transmitted is at a comparatively low frequency, zero bias driving is used, in which the bias current in the OFF state of the LD is perfectly 0. However, when the ON-OFF modulation frequency is increased, simple application of zero bias driving is difficult for the following reason. Assuming a life of carrier decided by the LD to be used as xcfx84, a threshold value of a current in the LD as Ith, the DC bias current to be continuously applied even while the LD is not in operation as Ib, and the driving pulse current amplitude while the LD is in operation as Ip, it is known that the delay time Td from the time when the current is injected to the time when the laser oscillation of the LD takes place is given by the following formula.
Td=xcfx84xc3x97In(Ip/(Ip+Ibxe2x88x92Ith))xe2x80x83xe2x80x83(1) 
Since xcfx84 is generally on the order of nano seconds, it is important to reduce the value of the logarithm term in the above formula to a value as small as possible such as 0.1 or less, when a signal transmission speed of 100 Mb/s or more is required. To satisfy Ib=0 which is a perfect zero bias condition, as well as to realize a value of logarithm term of 0.1 or less, it is necessary to reduce the ratio Ip/Ith to 0.1 or less, that is, to make the value of Ip extremely larger than 10 times of the value of Ith. In other words, the pulse driving current amplitude must be set to a value larger than the value for obtaining the necessary laser intensity amplitude. A problem thus arises that the current driving capacity of the LD driving circuit must be made higher than the value to be required at its minimum so as to obtain the necessary transmission optical signal intensity and that, as a result, the power consumption is increased at the same time.
On the other hand, in the pseudo-zero bias driving system, in which the DC bias current Ib is always applied which is slightly smaller than the threshold value Ith, the ratio Ip/(Ithxe2x88x92Ib) can be set easily to 10 or more, which is more advantageous even if Ip itself is not made large so much. Namely, when the pseudo-zero bias driving system is used, the delay time is reduced, and the high frequency operation is ensured, as well as a large light ON/OFF ratio is obtained and low power consumption can be realized at the same time.
However, a problem still remains that it is difficult for the pseudo-zero bias driving system to control Ib. The reason is that it is known that the threshold value of current Ith at an optional temperature Ti is expressed by the following approximate value, using an intrinsic characteristic temperature T0 of the LD to be used and a threshold value Is when the reference temperature T=Ts:
Ith=Isxc3x97exp((Txe2x88x92Ts)/T0)xe2x80x83xe2x80x83(2) 
Exhibiting a characteristic that Ith greatly changes with non-linearly for a temperature change. For example, the value T0 of an InP series LD is several tens to 100, so that the threshold change is close to several times to 10 times for a temperature change of 100 degrees. Further, even in a case of a GaAs series LD which is knnwn to have a characteristic of comparatively low temperature dependence within the range from the room temperature to about 70xc2x0 C., a constant term of Ic may be added to the right side of Formula (2) to increase accuracy in the approximation within the temperature change ranging from xe2x88x9240xc2x0 C. to 100xc2x0 C. It is known that thus compensated value of T0 shows the same value as that of an InP series element. In consideration of the above, it is essential for the DC bias generating circuit itself to have large temperature dependence in the same way as with Ith to make the value of Ib for realizing a pseudo-zero bias follow Ith and keep the difference between them to be substantially constant.
In the art prior to the disclosure of Japanese Patent Application Laid-Open 11-103108 applied by the applicant and shown in FIG. 1, a simple bias current generating circuit which can accurately follow temperature changes of the threshold current Ith and can be applied to LDs having various different characteristics was not realized. For example, a system for searching for the inflection point, at which a DC bias current is fixed, in the neighborhood of the threshold current by checking the differential value of a DC bias current, and a system for monitoring the intensity of actually emitted light of the LD, which is fed back to a DC bias are known as a compensation system of threshold bias current of LD. These systems require a large-scale detecting and feed backing circuit, so that it is almost impossible to apply them to uses requiring a compact one-chip IC such as LD driving circuits for the optical data link.
On the other hand, it is known that not only the threshold value Ith but also the light intensity emitted of the LD have temperature dependent characteristics, which is expressed by an exponential function decreasing with temperature, assuming the characteristic temperature T0xe2x80x2 as a constant. Since the value of T0xe2x80x2 is large compared with T0 and is generally several hundreds, temperature compensation of the light intensity emitted is often required even though it changes slightly unlike the case with the threshold value. In the conventional optical communication, an APC (automatic power control) circuit for keeping the light intensity emitted from the laser constant is used to keep the magnitude of a optical transmission signal constant and to prevent deterioration in quality of the signal transmitted. A large-scale APC circuit for monitoring a part of output of the LD by a phase detector (PD) and feeding it back is generally used to realize severe control by active feedback.
Further, due to an improvement in uniformity and stability of physical characteristics of LD, in recent years, a feed forward type stabilizing circuit is used on a assumption that the temperature dependent characteristic of LD is regarded as almost constant. Namely, as a method for a simple temperature compensation of the light intensity emitted from LD, stabilization using a control system for applying passive feed forward is adopted.
Such a temperature compensation system of light intensity emitted from the LD in the feed forward type APC circuit is exemplary shown in Japanese Patent Application Laid-Open 3-21493.5 and Japanese Patent Application Laid-Open 8-139410. Following systems have been designed by confirming the characteristics of LD beforehand.
(1) a system for making rough approximation using the temperature dependence of a diode in an IC,
(2) a system increasing the approximate accuracy comparatively by selecting a thermistor,
(3) a system for approximating the characteristics by broken lines, switching several kinds of resistors and
(4) a system for storing LD characteristics in a memory and severely adjusting them using a D-A converter.
Further, as disclosed in Japanese Patent Application Laid-Open 7-76287, there is a system combining a voltage source obtained by deforming a band gap reference power source, an emitter follower, and a current feedback amplifier. However, problems are remained in the above systems that compensation characteristics for temperature change are insufficient, that the temperature range is limited, that there are many parts for adjusting the characteristics, and that the adjustment itself is complicated. Further, even in a system having few of the above problems that there are disadvantages that a complicated circuit is required instead, that the chip area is increased, and that it is applicable to a specific LD but is not applicable to an LD having a slightly different characteristic temperature.
Japanese Patent Application Laid-Open 11-103108 relating to the invention of the inventor of this application shown in FIG. 1 indicates a strong improvement for the above various problems.
As shown in the drawing, the reference voltage generating circuit 1 generates a reference voltage Vref by using, for example, a band gap voltage reference power source of a basic circuit construction. A voltage generating circuit 2 generates a reference bias voltage Vg, which is higher than Vref the voltage and is as stable as the voltage Vref. The value of Vg is slightly lower than the voltage for generating a current Is of a desired value at the reference temperature Ts and is selected at a value, which is lower by the voltage appeared across a reference resistor RG supplied with a current given at an ambient temperature.
A voltage dividing circuit 4 and a current generating circuits 5 are provided.
The voltage dividing circuit 4 is composed of resisters R1, R2. The current generating circuit is composed of an emitter grounded type amplifier circuit including an npn transistor Q1 and a resistor R3 connected to an emitter electrode thereof. The current generating circuit 4 provides a collector electrode of the transistor Q1 with a current varying with an exponential function depending on a variation in an ambient temperature. The reference voltage is supplied to a base electrode, by being reduced and divided by the resistors R1 and R2. The current mirror circuit 6 designed for the purpose of ejecting a current from the plus side of the power source can realize an operation having good linearity even when it has a simple construction composed of of PNP transistors Q2 and Q3 which are complementary to Q1 and a resistor R6. The current reversed by the current mirror circuit 6 is supplied to the reference resistor RG, which generates at its both ends a voltage, which is a sum of a component voltage Vg independent from the temperature and a component voltage strongly dependent on temperature. Since an output impedance of the voltage generating circuit 2 at the end of the current-voltage conversion resistor RG, into which the current is supplied from the current mirror circuit 6, is almost as large as the value of RG, the output voltage is supplied via the buffer amplifier circuit 7 having a gain of 1, thereby lowering the output impedance.
The output voltage of the buffer amplifier circuit 7 is supplied to a base electrode of a transistor Q4 of an output amplifier 8, which is a current feedback type amplifier having a resistor R4 connected to an emitter electrode. An output current is obtained from a collector electrode of the transistor Q4, which is connected to the load.
FIG. 2 comparatively shows the results of the simulation of DC bias current and its approximated exponential function for a design example using a Si bipolar. It is found that the simulation results satisfactorily correspond to the exponential offset function of Formula (1) within the temperature range of 100xc2x0 C. with an error range of 0.2 mA or less, just by setting the characteristic temperature T0 within the range from 45xc2x0 C. to 58xc2x0 C. and by changing the RG resistance in three kinds. However, the method does not cover sufficiently the temperature range, although not shown in the drawing, of less than 0xc2x0 C., which is required for communication industry. In addition, an approximation error of about 0.2 mA cannot be avoided yet, although it is particularly improved compared with the previous method. Recently, the development of the LD having lower threshold value has been promoted, where a current of several mA is generally obtained. However, it is difficult to compensate the DC bias always up to {fraction (1/10)} or less of the threshold current due to an accuracy of the approximation of the generated current, thereby bringing a limitation in lower power consumption.
As described above, the pseudo-zero bias driving system requires a circuit for generating a DC bias current for faithfully following the temperature dependent characteristic of the threshold current of LD changing with great dependency on the temperature on an exponential function basis. The reason for it is that the system keeps flowing a fixed DC bias current, which slightly smaller than the threshold value in an LD so as to reduce the oscillation delay time, to ensure the high frequency operation and to ensure a large ON/OFF ratio at the same time to realize a high-speed optical data link. However, in the prior art, it is difficult to realize such a DC bias current generating circuit. Moreover, the temperature compensation can be applied only to an LD having a specific characteristic and a large-scale detection-feedback circuit is required. Thus, a problem arises that the cost is increased on one hand and miniaturization is essentially difficult on the other.
Further, in the prior feed forward type APC circuit, various systems for executing temperature compensation of the light intensity emitted of the LD are proposed. However, problems aroused in the above systems that compensation characteristics for temperature change are insufficient and the temperature range is limited, that there are many parts for adjusting the characteristics, that the adjustment itself is complicated, and that a complicated circuit is required. Thus the chip area is increased and is applicable to a specific LD but is not applicable to an LD having a different characteristic temperature.
Therefore, the present invention was developed to solve the above conventional problems and is intended to provide a temperature dependent constant-current generating circuit, which is small in size with a low-price, which faithfully follows temperature changes of a threshold current of an LD, which generates a DC bias current having particularly excellent in the accuracy in the temperature compensation characteristic, which can be widely applied to an LD driving circuit of a optical subscriber system of FTTH or a optical data link, and which can be applied also to temperature compensation of the intensity of the output from the LD.
To solve the above problems, the temperature dependent constant-current generating circuit according to the present invention has a stabilized voltage generating circuit including a reference voltage source for supplying a predetermined reference voltage, a plurality of voltage dividing circuits for dividing the reference voltage, a plurality of current amplifier circuits each of which includes a transistor whose base or gate is connected to an output terminal of the voltage dividing circuit and whose emitter or source is grounded, a current mirror circuit for providing an output current flowing in an opposite direction having a same magnitude as a composite output currents of the current amplifier circuits, a current-voltage conversion resistor, which is provided with the output current of the current mirror circuit, thereby generating a voltage proportional to the output current of the current mirror circuit, and which add the voltage proportional to the output current of the current mirror circuit to an output voltages of the stabilized voltage generating circuit, and an output amplifier circuit to which the added voltages generated by said current-voltage conversion resistor are supplied. The output amplifier circuit is composed of an output transistor, having a base or gate electrode, an emitter or source electrode and a collector or drain electrode. The added voltages generated by the current-voltage conversion resistor are supplied to the base or gate electrode. A feedback resistor is connected to an emitter or source electrode. An output current is taken out from the collector or drain electrode of the output transistor.
The reference voltage is kept constant against a variation in a supply voltage or in an operation temperature, while the stabilized voltage generating circuit is set to a voltage which is higher than the reference voltage, but slightly lower than the output voltage of the constant-current generating circuit for generating an output current of a desired value at the reference temperature. Namely, the output voltage of the stabilized voltage generating circuit is lower by the voltage generated across the current-voltage conversion resistor by the current provided by the current mirror circuit depending on the temperature at use.
More specifically, the output of the stabilized voltage generating circuit, which is independent from a temperature, is applied to one end of the current-voltage conversion resistor. On the other end of the current-voltage conversion resistor, a voltage, which is converted from the current generated from the current mirror circuit by the current-voltage conversion resistor and is dependent on a temperature, is generated in addition to the output voltage of the stabilized voltage generating circuit. In the current amplifier circuits, on the other hand, a voltage obtained by dividing the reference voltage by the voltage dividing circuit is added to the base or gate of the transistor. A current, the magnitude of which varies depending on a temperature with an exponential function is taken out from the collector or drain of the transistor. The output current flows into the voltage conversion resistor from the other end through the current-miller circuit.
As described above, a voltage is generated with which an output current of a desired value is generated from the constant-current generating circuit at the reference temperature and with which a current varying with an exponential function depending on a temperature other than the reference temperature is generated at the end of the current-voltage conversion resistor, into which the current from the current mirror circuit is flowing. Therefore, when the voltage generated at the current input end of the current-voltage conversion resistor is applied to the base or gate of the output transistor of the output amplifier circuit via the buffer amplifier circuit, an output current is taken from the collector or drain of the output transistor, which has a desired value at the reference temperature, varying depending on the temperature with an exponential function, and which is required for temperature compensation. The output current is supplied to a load connected to the collector or drain of the output transistor.
In this case, the voltage generated at the end of the current-voltage conversion resistor is supplied to the output circuit through a buffer amplifier, which is an amplifier having a gain of 1 and reducing an input impedance to the amplifier circuit, thereby preventing the input voltage of the output amplifier circuit from being affected by variation of a load of the output circuit.
The constant-current generating circuit according to the present invention generates an output current having a desired current at the reference temperature and changing with an exponential function depending on a temperature variation. Therefore, when it is applied to a LD driving circuit, it is able to provide a LD with a DC bias current, which always follows temperature variations of the threshold value current of a LD and which is slightly smaller than the threshold value current with good preciseness. Therefore, the pseudo-zero bias driving can be realized, which is conventionally difficult to realize.
Further, the constant-current generating circuit according to the present invention adopts a multiple current source construction, in which two or more current generating circuits are used for deciding a non-linear characteristic of the output current, and the composite current of the current generating circuits is finally converted to a voltage. Accordingly, a temperature dependent constant current circuit is obtained, with which the approximation to the non-linear characteristic is made with higher preciseness and the characteristic temperature T0 can be changed in a wider range compared with the conventional single source construction, in which a single current generating circuit is used. As a result, a DC current for pseudo-zero bias can be brought very close to the laser oscillation threshold value and the excessive over-drive current to speed up an operation is unnecessary, thereby decreasing an amplitude of a driving current for pulse modulation. According to a rough estimation, the current consumption can be reduced to a half or less of the conventional one. Further, regardless of a single or the multiple current source construction, it is possible that a component of the output current depending on a temperature and an offset component independent from the temperature can be set independently by selecting the relation between a stabilized voltage of the stabilized voltage generating circuit and a voltage across the current-voltage generating resistor in a manner described above.
Further, the voltage dividing ratio of the voltage dividing circuit is decided by the absolute value of the reference voltage, the characteristics of the current amplifier circuit (for example, voltage between the base and the emitter of the transistor), the resistance of the current-voltage conversion resistor, and the temperature dependence (characteristic temperature) of the output current from the constant-current generating circuit. However, among them, the values other than the characteristic temperature can be considered as fixed values. The characteristic temperature of the output current, thus, can be adjusted optionally by properly adjusting the voltage-dividing ratio. Namely, when the reference voltage and current-voltage conversion resistance are decided, it is possible to change the temperature dependence (characteristic temperature) of the output current by adjusting the voltage-dividing ratio of the voltage circuit. Further, the temperature dependence (characteristic temperature) of the output current can be also adjusted by adjusting the current-voltage conversion resistance.
Furthermore, since a current mirror circuit having a current amplifying operation can be provided by designing the size of the transistor constituting the current mirror circuit on the output side is larger than the size of the transistor on the input side, the expected operation can be realized even when the output current of the source current amplifier circuit is small.
In the temperature dependent constant-current generating circuit according to the present invention, except the current mirror circuit, the current amplifier circuit including the transistor, in which base or gate electrode is connected to the output terminal of the voltage dividing circuit and emitter or source electrode is grounded may be used as a current discharge type circuit. Thus, the current-voltage conversion resistor may be connected to the output terminal of the current discharge type current amplifier circuit.
Furthermore, according to the present invention, an amplifier circuit of an emitter follower or a source follower circuit, in which a collector or drain electrode is grounded, may be inserted between the output terminal of the buffer amplifier circuit and a base or gate electrode of the output amplifier. Thus, with the current amplification function of the added amplifier circuit, the stability of the output amplifier circuit is maintained and the output current can be increased. Since the amplifier circuit added varies the temperature dependent characteristic of the output current, the temperature dependence of a voltage generated at the current-voltage conversion circuit must be optimized again.
Further, an array output type constant-current generating circuit can be prepared and in this case, each added amplifier circuit may be equipped with an isolation function between the channels of array output by providing a plurality of the combined circuits including the amplifier circuit inserted between the buffer amplifier circuit and the output amplifier circuit.